1. Field of the Invention
The present invention relates to a fiber Raman amplifier using a microstructured fiber and to a microstructured fiber suitable for Raman amplification. In particular, the present invention relates to a lumped fiber Raman amplifier and to a microstructured fiber suitable for a lumped fiber Raman amplifier.
2. Description of the Related Art
Fiber Raman amplifiers have been attracting a great attention, because of their capability to increase the transmission capacity. Raman amplifiers offer several advantages, such as a low noise, a greater flexibility in choosing the signal wavelength and a flat and broad gain bandwidth. The greater flexibility in choosing the signal wavelength mainly depends on the fact that the Raman peak of a material, exploited for the amplification of the signal, is dependent practically only on the pump wavelength, differently from what happens for example in erbium-doped fiber amplifiers, in which the choice of the signal wavelength is restricted by the stimulated emission cross-section of erbium. The broad gain bandwidth of Raman amplifiers can be much enlarged, for example by using multiple pump sources. Such a broad gain bandwidth may represent a way to extend the usable optical bandwidth outside the conventional C-band and the extended L-band of the erbium-doped fiber amplifiers. Lumped Raman amplifiers may also play an important role to compensate for not only the fiber attenuation but also losses of other optical components, such as connectors, switches, splitters and so on.
Up to now, dispersion compensating fibers (DCF) or, more generally, fibers having a high non-linearity have been proposed for realizing fiber Raman amplifiers. For example, T. Tsuzaki et al., in “Broadband Discrete Fiber Raman Amplifier with High Differential Gain Operating Over 1.65 μm-band”, OFC2001 MA3-1, describe a high differential-gain (0.08 dB/mW), low-noise (<5.0 dB), broadband (30 nm) and flat-gain (<±1 dB) fiber Raman amplifier operating over the 1.65 μm-band which employs a low-lossy highly nonlinear fiber (HNLF) and a broadened pump light source. The fiber has a transmission loss of 0.49 dB/km at 1.55 μm and of 0.47 dB/km at 1.65 μm, an effective area Aeff of 10.10 μm2 at 1.55 μm, a dispersion of 1.79 ps/nm/km at 1.55 μm, a Δn of 3.10% and a ratio gR/Aeff of 6.50·103 1/Wm. FIG. 1 of the article shows an attenuation at a wavelength of 1450 nm of about 0.7 dB/km.
In order to evaluate the Raman amplification characteristics free from the influence of the fiber length, the authors of the article use the following figure of merit:FOM=(gR/Aeff)/αp  [1]where (gR/Aeff) and αp are the Raman gain coefficient and the fiber attenuation at the pump wavelength. With 1450 nm pumping, the FOM is estimated to be 9.3 1/W/dB. If the pumping wavelength is set to 1550 nm, the FOM becomes as high as 13.2 1/W/dB.
Recently there has been a great interest in fiber structures that incorporate numerous air holes surrounding a solid silica core. These air-silica microstructured fibers, similar to earlier single-material optical fibers, guide light within the core as a result of the index difference between the silica core and the air-silica cladding. Microstructured fibers are also known in the art as “photonic crystal fibers” or as “holey fibers”.
For example, J. A. West et al., “Photonic Crystal Fibers”, ECOC 2001, Th A 2.2, review various types of air-silica microstructure fibers, such as effective-index photonic crystal fibers (EI-PCF), air-clad core fibers and photonic band-gap fibers (PBGF). EI-PCFs are typically made from a hexagonal lattice of circular air columns in which the periodicity is relatively uniform. Losses as low as 2.6 dB/km are reported for this kind of fiber by the authors. Air-clad core fibers contains only a single ring of holes. The justification for removing the outer layers of holes is that in the limit in which the wavelength is large compared to the distance among the holes Λ, the fiber behaves very much like an equivalent step-index fiber. When the air holes are very large this fiber becomes essentially a silica rod in air and thin silica struts, whose purpose is simply to support the core, can replace the periodic lattice of air holes. Typical losses of 5–10 dB/km are reported for this kind of fibers by the authors. PBGF relies completely on the physics of photonic band-gaps for waveguiding and allows true guidance in low index cores.
Microstructured fibers can differ significantly from conventional optical fibers, allowing for properties that cannot be realized in standard fibers.
For examples J. K. Ranka et al., in “Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm”, Optics Letters, Vol. 25 No. 1, pag. 25–27 (2000) disclose that air-silica microstructure optical fibers can exhibit anomalous dispersion at visible wavelengths. They exploit this feature to generate an optical continuum 550 THz in width, extending from the violet to the infrared, by propagating pulses of 100-fs duration and kilowatt peak powers through a microstructure fiber near the zero-dispersion wavelength.
Microstructured fibers with doped cores have been proposed. For example, patent U.S. Pat. No. 5,802,236 to Lucent Technologies Inc. discloses a fiber having a solid silica core region that is surrounded by an inner cladding region and an outer cladding region. The cladding regions have capillary voids extending in the axial fiber direction, with the voids in the inner cladding region having a larger diameter than those in the outer cladding region. Applications disclosed in '236 patent of the microstructured fiber comprise all-optical non-linear Kerr switching in a fiber having a photosensitive core, using a Bragg or long period grating. Such fiber exemplarily has a Ge, B, or Sn-doped core.
In another example, C. E. Kerbage et al., in “Experimental and scalar beam propagation analysis of an air-silica microstructure fiber”, Optics Express, Vol. 7 No. 3, pag. 113–121 (2000) study the higher order guided modes in an air-silica microstructure fiber comprising a ring of six large air-holes surrounding a Germanium doped core. They characterize the modes experimentally using an intra-core Bragg grating.
Microstructured fibers can reach high optical nonlinearity. The large refractive index contrast between silica and air means that it is possible to confine light to transverse modes with a dimension of the order of the wavelength of light, meaning that such fibers have an effective nonlinearity per unit length 10–100 times higher than that of conventional silica fiber. This characteristic can be advantageously exploited in order to reduce the length/power levels required for devices based on non-linear effects.
Raman amplification in a microstructured fiber has been proposed by J. H. Lee et al., in “A holey fiber Raman amplifier and all-optical modulator”, ECOC 2001, Th A 4.1, who demonstrate the use of a short length of highly nonlinear holey fiber to obtain strong L-band Raman amplification. Using a 75 m long holey fiber with an effective area of 2.85 μm2 they obtain internal gains of over 42 dB and a noise figure of about 6 dB at 1640 nm. The loss of the fiber is 40 dB/km. The obtained gain efficiency is 6 dB/W. Further, the authors estimate the Raman gain coefficient gR to have a value of 7.6·10−14 m/W.
The Applicant observes that a gain efficiency of 6 dB/W means that in order to realize an amplifier having an internal gain of 20–25 dB, a pump power of more than 3–4 W should be used. This makes the fiber described in the Lee et al.'s article quite unpractical for a real installation in a telecommunications system.
The Applicant has faced the problem of realizing a Raman amplifier using a microstructured fiber, capable of reaching high gain with low pump power requirements. The amplifier should preferably have a low noise figure. The Applicant has perceived that, in order to have high Raman gain efficiency, that is high gain with low pump power, the microstructured fiber should have a high figure of merit for Raman amplification, according to formula [1].
The Applicant has observed that by using the formula [1] given above for calculating the FOM for Raman amplification of the fiber described by Lee et al. in their article cited above, a value of 0.67 1/W/dB would be obtained, that is a very low value. According to the Applicant this is mainly due to the high attenuation (40 dB) of the microstructured fiber. The Applicant has however recognized that even if the attenuation of a microstructured air-silica fiber were lower, the figure of merit for Raman amplification of an air-silica fiber would be at most comparable with the figure of merit for Raman amplification of a conventional dispersion compensating fiber or of a conventional highly nonlinear fiber. By applying the formula [1] given above, a “best” value of about 10 1/W/dB would be obtained by considering a gR of 7.6·10−14 m/W, a very low attenuation of the microstructured fiber at the pump wavelength of 2.6 dB/km and an effective area of 2.85 μm2. That is, even by considering a very low attenuation of the air-silica microstructured fiber, such fiber would have a figure of merit for Raman amplification at most equal to the figure of merit obtainable with conventional dispersion compensating or highly nonlinear fibers. According to the Applicant, this is due to the higher attenuation of the microstructured fibers, that in the best cases up to now can reach values of about 2.6 dB/km with respect to values lower than 0.5 dB/km for the conventional fibers. The “best” result of the figure of merit of an air-silica microstructured fiber given above cannot be considered much satisfactory, as the manufacturing of an air-silica fiber having a very low attenuation is rather complicated with respect to the manufacturing of a conventional fiber. Further, such result would be obtained by using microstructured fibers having very low effective areas (for example 2.85 μm2 in the example reported in Lee et al. article), giving rise to problems in the coupling of the pump radiation and of the optical signal in the microstructured fiber for amplification.